Liposomes in which fluorescent compounds have been included by sonication have been incubated with myeloma tumor cells, phagocytic cells, and various other lymphoid cell populations. Use of specific antigens in the liposome membrane has permitted quantification of surface immunoglobulin on MOPC 315 murine myeloma cells by fluorescence-microscopy, by fluorometry and by use of the fluorescence activated cell sorter. Inclusion of cytotoxic drugs within liposomes has shed light on the relationship between surface binding and drug-delivery to the cytoplasm. Current data suggest enhanced cytotoxic effect when drugs are liposome-bound, in cell populations which can incorporate these liposomes.